The present invention relates to a rotatable filter assembly, primarily intended for separation of fibres from a fibre suspension, which comprises a plurality of annular filter discs, formed by filter elements, which are mounted axially spaced, substantially parallel, on a horizontal drum, rotatable in bearings, driven by a motor, in a container for a fibre suspension that is to be separated, each filter element being formed by a hollow body, with walls made from straining cloth, which body communicates via a filtrate outlet, provided at the radially inner end, with an axial discharge tube in the drum, which is formed like a cylindrical drum with a grating-formed mantle wall, containing axial tubes, which form the discharge tubes, which communicate with the filter elements, a collecting hopper extending into the drum through one open end of it, with its opening, turned upwards, along all filter discs, carried by the drum, the interior of each filter element being kept, separately, at a pressure lower than that prevailing at the outside of it during at least part of the rotation of said drum.
Such filter assemblies are known from Swedish patent specification No. 7406315-7 and Swedish patent application No. 301082-7, the latter of which discloses a further development of the filter assembly, which is related to in the specification. For both types of filter assemblies it applies that there must be provided a subpressure within the filter elements during a certain part of the rotation turn of the filter assembly, whilst this subpressure shall be brought to cease during the rest of the rotation turn, that is to say during the so called taking off zone. Conventionally the subpressure is provided by the aid of an external vacuum source, that is to say a vacuum pump or a so called barometric leg, the communication between the discharge tubes and the vacuum source being provided by a valve or lock device. As vacuum pumps are expensive in installation and operation, normally barometric legs are used. In order to give a sufficient subpressure the barometric legs must be 6-9 meters long from the centre of the filter and furthermore they must be straight in order to operate. Furthermore, they only operate optimally at a certain flow rate. If the production rate is changed from the calculated one a lower subpressure or no subpressure at all is obtained. A further drawback with barometric legs is that a large amount of air is mixed into the filtrate when using them, which is unfavourable in a production process. According to the present invention this drawback is reduced.